The sky is not quite unchanging, just slow and far away. Jupiter, the largest planet in the solar system, catches a lot of the meteors, comets and other sky debris and in that way protects the inner planets such as Earth. Amateur astronomers working with professional astronomers have spotted two fireballs lighting up Jupiter's atmosphere this summer, marking the first time Earth based telescopes have captured relatively small objects burning up in the atmosphere of the giant planet. The two fireballs - which produced bright freckles on Jupiter that were visible through backyard telescopes - occurred on June 3, 2010, and August 20, 2010, respectively.

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Amateur backyard astronomy has many times found comets before big name observatories. Jupiter is easily observable by such telescopes and now these amateurs have some fame because of their Jupiter observations.

A new paper that includes both pros and amateurs, led by Ricardo Hueso of the Universidad del País Vasco, Bilbao, Spain, appears today in the Astrophysical Journal Letters. In the paper, astronomers estimate the object that caused the June 3 fireball was 8 to 13 meters (30 to 40 feet) in diameter. The object is comparable in size to the asteroid 2010 RF12 that flew by Earth on Wednesday, Sept. 8, and slightly larger than the asteroid 2008 TC3, which burned up above Sudan two years ago.

An impact of this kind on Earth would not be expected to cause damage on the ground. The energy released by the June 3 fireball as it collided with Jupiter's atmosphere was five to 10 times less than the 1908 Tunguska event on Earth, which knocked over tens of millions of trees in a remote part of Russia. Analysis is continuing on the August 20 fireball, but scientists said it was comparable to the June 3 object.

"Jupiter is a big gravitational vacuum cleaner," said Glenn Orton, a co-author on the paper and an astronomer at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "It is clear now that relatively small objects, remnants of the formation of the solar system 4.5 billion years ago, still hit Jupiter frequently. Scientists are trying to figure out just how frequently."

Jupiter has been called the Solar System's vacuum cleaner, because of its immense gravity well and location near the inner Solar System. It receives the most frequent comet impacts of the Solar System's planets. It was thought that the planet served to partially shield the inner system from cometary bombardment. However, recent computer simulations suggest that Jupiter does not cause a net decrease in the number of comets that pass through the inner Solar System, as its gravity perturbs their orbits inward in roughly the same numbers that it accretes or ejects them. This topic remains controversial among astronomers, as some believe it draws comets towards Earth from the Kuiper Belt while others believe that Jupiter protects Earth from the Oort Cloud.

A 1997 survey of historical astronomical drawings suggested that the astronomer Cassini may have recorded an impact scar from an asteroid or comet in 1690. In July 1994, over 20 fragments from the comet Shoemaker-Levy 9 collided with Jupiter's southern hemisphere, providing the first direct observation of a collision between two Solar System objects.

On July 19, 2009, an impact site was discovered at approximately 216 degrees longitude in System 2.[115][116] This impact left behind a black spot in Jupiter's atmosphere.

Orton and colleagues said this kind of discovery couldn't have been made without amateur astronomers around the world, whose observations of Jupiter provide a near round-the-clock surveillance that would be impossible to do with the long lines of scientists waiting to use the large observatory telescopes. Amateur astronomers, for example, were the first to see the dark spot that appeared on Jupiter in July 2009 as the result of an impact.

Wesley was visiting an amateur astronomer friend about 600 miles away in Broken Hill, and he set a digital video camera to record images from his telescope at about 60 frames per second. He was watching the live video on a computer screen at his friend's house when he saw a two-and-a-half-second-long flash of light near the limb of the planet.

"It was clear to me straight away it had to be an event on Jupiter," he said. "I'm used to seeing other momentary flashes in the camera from cosmic ray impacts, but this was different. Cosmic ray strikes last only for one frame of video, whereas this flash gradually brightened and then faded over 133 frames."

For three days afterward, Hueso and colleagues looked for signs of the impact in high-resolution images from larger telescopes: NASA's Hubble Space Telescope, Gemini Observatory telescopes in Hawaii and Chile, the Keck telescope in Hawaii, the NASA Infrared Telescope Facility in Hawaii and the European Southern Observatory's Very Large Telescope in Chile. Scientists analyzed the images for thermal disruptions and chemical signatures seen in previous images of Jupiter impacts. In this case, they saw no signs of debris, which allowed them to limit the size of the impact object.

Based on all these images, and particularly those obtained by Wesley and Go, the astronomers were able to confirm the flash came from some kind of object — probably a small comet or asteroid — that burned up in Jupiter's atmosphere. The impact object likely had a mass of about 500 to 2,000 metric tons (1 million to 4 million pounds), probably about 100,000 times less massive than the object in July 2009.

The second fireball, on Aug. 20, was detected by the amateur Japanese astronomer Masayuki Tachikawa and later confirmed by Aoki Kazuo and Masayuki Ishimaru. It flashed for about 1.5 seconds. The Keck telescope, observing less than a day later, also found no subsequent debris remnants. Scientists are still analyzing this second flash.

Although collisions of this size had never before been detected on Jupiter, some previous models predicted around one collision of this kind a year. Another model predicted up to 100 such collisions. Scientists now believe the frequency must be closer to the high end of the scale.

"It is interesting to note that whereas Earth gets smacked by a 10-meter-sized object about every 10 years on average, it looks as though Jupiter gets hit with the same sized object a few times each month," said Don Yeomans, manager of the Near-Earth Object Program Office at JPL, who was not involved in the paper. "The Jupiter impact rate is still being refined and studies like this one help to do just that."

Comets. meteors and comets are constantly present in the solar system. Many such impacts must be occurring with most of them unobserved. Amateurs astronomers are vital in detecting such impacts and aiding in the overall study of how the solar system handles such impacts and their frequency.